| Summary: | This study describes the application of multi-walled carbon nanotubes that were nitrogen-doped during their synthesis (N-MWCNTs) in melt-mixed polypropylene (PP) composites.<b> </b>Different types of N-MWCNTs, synthesized using different methods, were used and compared. Four of the five MWCNT grades showed negative Seebeck coefficients (<i>S</i>), indicating n-type charge carrier behavior. All prepared composites (with a concentration between 2 and 7.5 wt% N-MWCNTs) also showed negative <i>S</i> values, which in most cases had a higher negative value than the corresponding nanotubes. The <i>S</i> values achieved were between 1.0 μV/K and −13.8 μV/K for the N-MWCNT buckypapers or powders and between −4.7 μV/K and −22.8 μV/K for the corresponding composites. With a higher content of N-MWCNTs, the increase in electrical conductivity led to increasing values of the power factor (<i>PF</i>) despite the unstable behavior of the Seebeck coefficient. The highest power factor was achieved with 4 wt% N-MWCNT, where a suitable combination of high electrical conductivity and acceptable Seebeck coefficient led to a <i>PF</i> value of 6.1 × 10<sup>−3</sup> µW/(m·K<sup>2</sup>). First experiments have shown that transient absorption spectroscopy (TAS) is a useful tool to study the carrier transfer process in CNTs in composites and to correlate it with the Seebeck coefficient.
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